Not applicable.
The present invention is directed to printed wiring boards having recesses, or other non-conductive surfaces, made electrically conductive by applying a coating containing carbon, preferably graphite, to an initially nonconductive through hole or other surface.
The prior art, for example U.S. Pat. No. 3,224,986, discloses water-insoluble, water-insensitive polymers made by (1) forming a polyamide, (2) reacting the polyamide with epichlorohydrin to make a cationic, water-soluble, thermosetting resin; and (3) reacting the resin with a water-soluble polymer, such as carboxymethylcellulose or others, in water at a low pH. Related disclosure may be found in U.S. Pat. Nos. 3,658,873; 3,049,469; 3,962,159; 3,917,894; 4,037,017; 4,152,199; 2,926,116; 2,926,154; 3,332,834; 3,592,731; and 3,763,060. All the patents listed in this paragraph are hereby incorporated by reference. These patents assert that various polysaccharides, such as starches and carboxymethylcellulose, react with polyamide-epichlorohydrin resins to form water-resistant coatings.
Additional background information about conductive coatings for through holes may be found in U.S. Pat. No. 5,690,805. That patent is hereby incorporated by reference in the present disclosure.
One aspect of the present invention is a method of applying an electrically conductive carbon coating to a non-conductive surface.
A substrate is provided having at least one non-conductive surface. An example of such a substrate is the wall of a through hole or via drilled or otherwise formed in the non-conductive substrate for a printed wiring board.
A conditioning agent is provided. The preferred conditioning agent contains a substantive cationic conditioner, for example, a polyamide, more preferably a polyamide epichlorohydrin resin in one embodiment of the invention.
A liquid dispersion of electrically conductive carbon including a water-dispersible binding agent is also provided. The carbon dispersion has a mean particle size no greater than about 50 microns. The carbon particles in the dispersion preferably have a mean particle size of not greater than about 1 micron, particularly for a graphite coating.
The nonconductive surface to be made electrically conductive is first contacted with the conditioning agent to apply a film of a cationic substantive conditioner on the nonconductive surface. The carbon dispersion is applied to the conditioned surface in an amount and under conditions effective to form a substantially continuous, electrically conductive carbon coating.
Next, the conductive carbon coating is fixed on the (formerly) nonconductive surface by applying an aqueous acid. (xe2x80x9cFixingxe2x80x9d is defined below in the detailed description.) Typically, fixing is carried out after the carbon dispersion is applied, without drying the carbon coating first. The fixing process removes excessive carbon composition deposits, and thus smooths the carbon coating on the recess surfaces by eliminating lumps and by making the coating more uniform and adherent.
As used herein, a xe2x80x9cuniformxe2x80x9d coating is one essentially free of excess conductive coating composition build-up, particularly at the opening or openings of a recess, so the coating has a substantially uniform thickness at the mouth and in the interior of the recess, as viewed under a 50xc3x97 magnification of a cross-section of a recess after plating. Graphite and carbon black are referred to in this specification either together or separately as xe2x80x9ccarbon.xe2x80x9d A non-conductive surface can be any surface that does not conduct electricity to the necessary or desired degree for some practical application.
Another aspect of the invention is a printed wiring board. The printed wiring board includes at least two conductive circuit layers separated by nonconductive material. At least one recess in the nonconductive material has a nonconductive surface intersecting at least two of the conductive circuit layers.
An electrically conductive coating is provided on the nonconductive surface. This coating includes electrically conductive carbon having a mean particle size not greater than about 1 micron and a water-dispersible organic binding agent. The coating is electrically conductive, allowing electrical current to flow between the two conductive circuit layers. The conductive carbon coating can optionally be electroplated to provide a surface at least substantially free of visible voids. The electroplated surface can optionally be soldered to form electrical connections between the conductive circuit layers and the leads of electrical components, which is the typical construction of a complete printed wiring board.